Mortalities resulting from infections with Photobacterium damselae subsp. piscicida (formerly Pasteurella piscicida) cause the most significant losses in warm water marine aquaculture worldwide. The disease (pasteurellosis) has great economic impact in Japan, where it affects mainly yellowtail cultures, and in the Mediterranean area, due to the losses it causes in sea bream and sea bass farms. Antibiotic therapy is generally ineffective and undesirable due to its negative environmental impact. The development of a vaccine against this disease has been slow, predominantly as a result of the pathogen being facultatively intracellular, and therefore not generally exposed to immune defence mechanisms. Until now, vaccine research has focused on bacterins prepared from heat- or formalin-killed cells. A bacterin vaccine enriched in extracellular products (ECPs) called “DI21” has been commercialized in certain European countries. The degree of efficacy obtained with these bacterins is highly variable and the duration of protection is often short.
There is an unmet need in the field to deliver an inexpensive, easy to manufacture, and reproducibly effective vaccine against Photobacterium infection.